Both things are correct in the grammatical sense, but they have different meaning, and in the Nvidia case, "4.5x as fast" would have been correct, or they should have used the less impressive sounding "3.5x faster".

It might be easier to grasp if you use 1x. E.g. "it's 1x as fast" means it's exactly the same. You're applying the multiplier directly to get the new speed. 0.5x as fast means it's half as fast.

"It's 1x faster" means you add the result of the multiplication to the initial value. So it's twice as fast. 0.5x faster still means it's faster, you add 50% of the initial speed. This way you cannot really express that something got slower, except by using negative values, which might be rather confusing.

I think this might also work in most other languages originating in Europe.